Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 18

Cytokines which drive innate responses include TNFα,IL-1β, IL-6, CK-1, IL-8, IL-10, IL-11,
IL-12, IL-15, IL-17, IL-18, and type I and type II interferons. Such immune signals initi-
ate inflammation, induce chemotaxis of immune cells, and regulate inflammation, respiratory
burst and pathogen killing responses via induction of soluble molecules or the proliferation
and activation of natural killer cells and antiviral responses, preventing viral replication (see
Section 2.4).
Finally, the innate immune system, although being very robust in teleost fish, plays a role
in the development of a more pathogen- and antigen-specific response upon host re-exposure
to the pathogen. Thus, the cells and cytokines of the innate immune system have an important
role in the adaptive immune response. The characteristic effector function of cells such as
macrophages to phagocytose, process and present antigenic peptides to T cells, coupled with
the ability of cytokines to direct T cell differentiation, ensure that the teleost is capable of
mounting an antigen-specific adaptive immune response that effectively protects the host from
specific pathogen exposure.
2.3 ANTIGEN-SPECIFIC ADAPTIVE IMMUNITY
In an aquatic environment which is highly antigenic, teleost fish are the first vertebrates to
exhibit an adaptive immune system, where the response elicited is determined by type of
pathogen/antigen and route of entry into the host. They display all the characteristics under-
stood for adaptive immunity: response is antigen specific, slower to develop and displays
memory for enhanced or more rapid responses upon re-exposure to the same antigen/pathogen.
The teleost immune system must react to a wide array of pathogens such as bacteria, viruses,
fungi, protozoa and helminths, and this defence is influenced by temperature, pH, oxygen
tension and salinity; in other words the fish immune system and particularly the adaptive
response must be capable of functioning under a broad spectrum of environmental conditions
(Engelsma et al. 2003). Significant changes to these environmental parameters can result in
immune deviation or even immune suppression with grave effects on the host's ability to fight
off the infectious organism (reviewed in Tort et al. 2004). The presence of an adaptive/specific
immune response is confirmed by the expression of MHC (gene products for IA, β 2 m, IIA
and IIB), TCR and immunoglobulin (Ig) as well as the presence of helper and cytotoxic T
cells and differential subpopulations of B cells (Stet et al. 1996; Miller et al. 1998; Scapigliati
et al. 1999; Nakanishi et al. 2002). The presence of an adaptive response, however, does not
detract from the importance of the innate defences which, although not antigen/pathogen spe-
cific, are defence mechanisms that are mounted by the host in a matter of minutes/hours post
infection. The adaptive immune system, by its very nature of developing more sophisticated
antigen-specific defences, and taking into consideration the low temperature environment of
these fish, is much slower to initiate and may take weeks to months to develop. This makes
the innate immune system of utmost importance if a host is to survive primary stages of infec-
tion with a pathogen and be able to tailor a specific adaptive response that facilitates pathogen
killing and clearance.
Our understanding of functional development and elicitation of adaptive responses in teleost
fish is predominantly derived from molecular and immunohistochemical characterization of
adaptive markers and by inference from the wealth of understanding of mammalian adaptive
systems. With regards to characterization of effector cells of the adaptive immune system,